Fat emulsions have been widely used components of total parenteral nutrition because of their high caloric value, their isotonic properties, and their contents of essential fatty acids. A wide spectrum of patients are likely to receive the fat emulsions. These consist of patients with any clinical condition which prevents a satisfaction of caloric needs through the enteric route. The ages of patients who may be treated with fat emulsions range from premature infants to geriatric patients close to the end of their natural life span.
Triglycerides of fat emulsion are hydrolyzed by lipases in the plasma into metabolites which are among the biologically most toxic compounds on earth. These metabolites are the free or unesterified fatty acid molecules (FFA). The characteristics of the FFA have been reviewed in a recent article of the author of this invention and co-worker (Fertility and Sterility, Volume 45, pp. 273-279, Feb. 1986). FFA molecules are known to be toxic to red cells, white cells, heart cells, brain cells, and a variety of other cell types.
Despite their toxic properties the FFA molecules are essential for several physiologic systems. They are the major metabolite through which energy is obtained from dietary and physiologic fat molecules, and they are incorporated into phospholipids and other molecules to form components of cell membranes. In addition, they participate in defense mechanisms and in regulation of immune responses.
The coexistence of life with toxic FFA molecules is possible because of homeostatic mechanisms which neutralize fatty acid toxicity and regulate the concentration of the free fatty acid molecules. Under normal conditions the plasma contains an average FFA concentration of 0.4-0.8 mEq/l with a range of 0.315-1.218 mEq/l. The major plasma component which neutralizes fatty acid toxicities is albumin. The ability of albumin to efficiently neutralize FFA toxicity in the plasma seems to be limited to about two free fatty acids per albumin molecule. The normal range of FFA/albumin ratios is 0.5-2. When the ratio exceeds 2 the toxic effects of fatty acids become evident in a variety of essential physiologic systems.
For example, an FFA/albumin ratio of 2-3 inhibits chemotaxis of neutrophils by about 50%, makes platelets more susceptible to aggregation, and cause morphologic and immune changes in the red cells. A ratio of 3 was demonstrated to depress the contractility of rat heart preparations, a ratio of 4 depresses phagocytosis and bactericidal abilities of neutrophils, and displaces bound bilirubin from albumin molecules. This increases the risk of kernicterus in infants. A ratio of 5 changes enzymatic patterns in perfused heart muscle. A ratio of 6-7 causes heart beat abnormalities in patients with myocardial infarctions.
Studies of fluctuations of FFA during administration of parenteral fat emulsion have indicated that free fatty acids may reach levels known to be toxic to cells. For example, Andrew, et al. (Journal of Pediatrics, volume 88, page 273-277, 1976), infused infants within the first 48 hours of birth with 1 g/kg of Intralipid over a four hour period. This resulted in a mean FFA/albumin ratio of 10.75.+-.2.19 in 7 small for gestational age infants. In 10 appropriate for gestational age infants of less than 33 weeks gestational age the FFA/albumin ratio was 4.99.+-.0.37. In 10 appropriate for gestational age infants over 33 weeks the FFA/albumin ratio was 3.88.+-.0.37. Such high ratios are most likely to result in clinically significant side effects.
Factors which determine the release and removal of free fatty acid are numerous and complex. In the severely ill patient suffering from metabolic distortions, the effect of parenteral fat emulsion upon FFA levels may be especially hard or impossible to predict. A monitoring procedure which can be used to adjust the rate of fat emulsion infusion while the fat emulsion is being infused would constitute the most effective means for the prevention of excess accumulation of free fatty acids.
Standard biochemical methods currently available require specialized biochemical laboratories. The results are usually available to the clinician several days after the blood has been withdrawn from the patient. By that time the damage caused by the toxic concentrations of the free fatty acids had already taken place. This may result in additional suffering or even the death of the patient.
In addition, a minimum of several ml of blood are required for the biochemical analysis. This could rapidly deplete the small blood reserves of a premature or newborn infant who are receiving fat emulsion therapy.
One objective of the present invention is to devise an improved monitoring method which can utilize only one drop of blood to detect FFA toxicity. An additional objective of the present invention is to devise an FFA toxicity monitoring method which would yield results almost immediately after the blood has been withdrawn. A third objective of this invention is to devise a method which could detect FFA toxicity at very early stages of toxicity.
Essentially, the method described in this disclosure is based upon the observations that toxic free fatty acid/albumin ratios are reflected in the changes in red cell morphology. The red cells can therefore be used as sensitive indicators of FFA toxicity.